J. Am. Chem. SOC. 1993,115, 44014408
Intermolecular Proton-Deuterium Polarization Transfer in Magic Angle Spinning N M R Spectra: A New Spectroscopic Tool for Interfaces
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Nicholas Zumbulyadis' and James M. OReilly Corporate Research Laboratories Eastman Kodak Company Rochester, New York 14650-2132 Received February 1, I993 The combination of cross-polarization (CP) and magic angle spinning (MAS) I is a well-established method for obtaining highresolution NMR spectra of solids and has been used primarily in recording the spectra of spin 1/2 nuclei such as I T , 29Si,and 3iP. Some results involving the central transition of half-integer quadrupolar nuclei such as I7O and 29Al (I = 5 / ~ )have been reported,2J and the underlying theory has beendiscussed by Vega? Deuterium, on the other hand, is an integer spin nucleus (I= l), and its NMR spectrum has been used extensively to characterize the dynamics and orientation of polymers5-6 and biological macromolecules. These studies are based on the detailed analysis of the powder patterns (Pakedoublets) obtained from nonspinning samples using one- and two-dimensional NMR. In this communication we report for the first time an approach that combines deuterium NMR with cross-polarization and magic angle spinning. In the systems we have studied, cross-polarization is an intermolecular process. Hence, it gives rise to novel experimental opportunities for studying both the miscibility of polymer blends and polymer-diluent or guest-host interactions and for selectively accessing the interfacial region between protonated and deuterated phases by solid-state NMR. The broad, so-called Pake doublet observed in the static 2H NMR spectra (i.e., spectra obtained in the absence of mechanical sample spinning) of solids, micelles, and liquid crystalline materials is inhomogeneously broadened and will upon magic angle spinning give rise to rotational echoes in the 2H free induction Several recent papers have investigated 2H MAS spectra both t h e o r e t i ~ a l l yand ~ ~ ~experimentally ~ for the purpose of solids imaging,!I sensitivity enhancement,12resolution of inequivalent deuterium sites,I3 and 2H-13Cinternuclear distance determinations via rotational echo double resonance (REDOR).I4 With the exception of the latter, these experiments were run in the absence of proton spin decoupling, and in the case of spin imaging studies,' I the predicted dephasingl5 of the rotational echoes due to proton-deuteron dipolar interactions has been observed. We have observed cross-polarization between protons and deuterium under MAS conditions in several partially deuterated materials, solid solutions, and polymer blends. In the following, we report our results on a 20% blend of perdeuterated atactic (1) Schaefer, J.; Stejskal, E. 0. J . Am. Chem. Soc. 1976,98, 1031. (2) Blackwell, C. S.; Patton, R. L. J. Phys. Chem. 19884.88, 6135. (3) Walter, T. H.; Turner, G. L.; Oldfield, E. J. Magn. Reson. 1988, 76, 106. (4) Vega, A. Solid Srate Nucl. Magn. Reson. 1992,I, 33. (5) Spiess, H. W. Chem. Reu. 1991, 91,1321. (6) Jelinski, L. W. Annu. Reu. Mater. Sci. 1985,15, 359. (7) Mariq, M. M.; Waugh, J. S.J . Chem. Phys. 1979, 70, 3300. (8) Ackerman, J. L.; Eckman, R.; Pines, A. Chem. Phys. 1979,42, 423. (9)Ye, C.; Sun, B.; Maciel, G. E. J . Magn. Reson. 1986,70, 241. (10) (a) Clayden, N. J. Chem. Phys. Leu. 1986,131,517.(b) Kristensen, J. H.; Bildsoe, H.; Jakobsen, H. J.; Nielsen, N. C. J. Magn. Reson. 1992,100, 437. (1 1) Giinther, E.; Bliimich, B.; Spiess, H. W. Chem. Phys. Lett. 1991,184, 251. (12) Schadt, R. J.; Dong,R. Y.; Giinther, E.; Bliimich, B. J . Magn. Reson. 1992,96,393. (13) Kim, A. J.; Butler, L. G. J . Magn. Reson. 1992,99,292. (14) Schmidt, A.; McKay, R. A.; Schaefer, J. J . Magn. Reson. 1992,96, 644. (15) Gruwel. M. L. H. Chem. Phys. Lett. 1992, 191, 621.
Figure 1. Deuterium MAS spectra of a blend of PMMA-dg (20%) in PMMA-hg. (a) 2H time domain spectrum following cross-polarization from protons with a contact time of 1.5 ms and a 46-KHz spin-lock radiofrequency field. The rotational echoes are separated by the rotor period (- 191 ps) and persist for over 6 ms. Avcrageof 13 000 transients. (b) 2H time domain spectrum following a 5 . 4 - p 90° pulse. Average of 2300 transients. (c) No signal is observed in pure PMMA-ds after averaging 6376 transients. The delay between pulses was 5 s for all spectra.
poly(methy1 methacrylate) (PMMA-d8) in protonated atactic PMMA (PMMA-hs). The characterization of the PMMA samples has been previously reported.16 The *Hfree induction decay obtained by a standard lH-ZHcross-polarizationsequence using spin-temperature alternation and a contact time of 1.5 ms is shown in Figure la. It consists of a series of rotational echoes (16) OReilly, J. M.; Teegarden. D.M.; Wignall, G. D. Mucromolecules 19as,i8,2747.
0002-7863/93/1515-4407$04.00/0Q 1993 American Chemical Society
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spaced by the sample rotation period (- 191 ~ s ) . It is identical to the single pulse excitation spectrum (Figure lb), only weaker due to inefficiencies in the cross-polarization process. N o crosspolarization was observed in a sample of PMMA-ds (Figure IC) because of the very low concentration and long spin-lattice relaxation time of any residual protons. It has been our experience that the spin-lattice relaxation of residual protons in perdeuterated glassy and crystalline systems is longer than that in the corresponding protonated materials. In polystyrene-ds, for example, the residual proton TI is longer than 15 s (Zumbulyadis and OReilly, unpublished results). Thus, IH-*H cross-polarization in such systems showed be viewed as a primarily intermolecular process unless the spectrum of the perdeuterated material by itself points to the contrary. We have measured the cross-polarization rate in the PMMA/ PMMA-ds blend system by monitoring the dependence of the third, fourth, and fifth rotational echo on contact time (cf. Figure 2). A value of 2.3 ms was determined for the cross-polarization time constant Tcp. We have determined that protonated and deuterated PMMA are miscible, as was assumed in the interpretation of small-angle neutron scattering experiments.16 Previous NMRdeterminations of polymer miscibility by intermolecular cross-polarization have relied on polarization transfer from the protons of one polymer to the I3C nuclei of another, perdeuterated, component.17 Such studies presume, however, that the typically weak l3C signals of the deuterated component are sufficiently resolved and can be distinguished from the stronger and potentially overlapping signals of the protonated species. This problem cannot be always allayed by a simple delayed decoupling experiment, since quaternary carbons and carbonyls and their associated spinning sidebands cannot be effectively suppressed in this manner. Our approach _ _ _ _ ~
(17) (a)Parmer, J. F.;Dic!&on, L. E F h i e n , J. C. W.; Porter, R X Macromolecules 1987, 20, 2308. (b) Gobbi, G.C.; Silvestri, R.; Russell, T. P.; Lyerla, J. R.; Fleming, W. W. J . Polym. Sci. Polym. Left. 1!?87, 25, 61.
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Figure 2. Proton-deuteriumcross-polarizationdynamicsof 20%PMMAd8 in PMMA-hs. The average of the normalized intensities of the third, fourth, and fifth rotational echois plotted as function of C P contact time.
circumvents this difficulty. The lack of an interfering deuterium background allows miscibility or, stated moregenerally, molecular proximity studies in systems where one of the components is present in very low concentrations. Such a situation arises in the technologically important case of block copolymer compatibilizers or in the biochemically important case of studying small amounts of a host in a guest matrix. While some of these experiments may also be possible through lH-l3C cross-polarization coupled with double isotopic labeling, e.g., simultaneous deuteration and 13C enrichment, our approach eliminates the need for such cumbersome syntheses. Finally, the well-known spectral editing potential of CP/MAS experiments could lead to improvements in solidstate deuterium NMR imaging.]]
